Chimney swift (Chaetura pelagica) COSEWIC assessment and status report: chapter 10
Limiting Factors and Threats
- Habitat Loss
- Chimney Sweeping During the Breeding Period
- Public Misconceptions About the Species
- Winter Range
- Weather Conditions
- West Nile Virus
The Chimney Swift is part of an aerial insectivore guild that includes swallows and nighthawks that is suffering significant population declines throughout the Americas. Of this guild, the Chimney Swift has had the most serious decline. The cause for this widespread decline is unknown, but likely involves impacts to insect populations through pesticides, habitat loss or some other factors.
One of the main causes of the decline in the North American Chimney Swift population seems to be the reduction in the number of nesting and roosting sites: large-diameter hollow trees, old abandoned buildings and suitable chimneys (Kyle and Kyle 1996; Driftwood Wildlife Association 2000; Cink and Collins 2002). This decline in suitable sites is projected to continue over the next few decades, with very few sites remaining 30 years from now.
The few chimneys still suitable for swifts are often swept in the summer, coinciding with the swift’s breeding period. In Texas, the Driftwood Wildlife Association has partnered up with the National Chimney Sweep Guild in order to educate professional chimney sweepers about the Chimney Swift’s plight and promote chimney maintenance outside of the breeding period (Kyle 1999).
Public misconceptions about the species have resulted in building owners’ intolerance for swifts nesting in their chimneys. They often cite fire risks as the reason for preventing Chimney Swifts from using their chimneys, even though such risks are non-existent. Roosting swifts cling to the chimney walls at night and leave in the morning. Nesting swifts build only one nest per site, and it is minuscule (approximately 10 cm long and 5 X 7.5 cm wide (Fischer 1958)). It is made of small twigs and often falls to the bottom of the chimney at the end of the season. As a result, there is no possibility of the nest blocking the chimney and causing a fire. Poor chimney maintenance is a much greater danger. However, it must be recognized that the rearing of Chimney Swifts can be quite noisy, forcing homeowners to intervene. Also, some people confuse swifts with more problematic bird species such as the European Starling, which prompts them to remove the nests.
Since the Chimney Swift depends mainly on insects for its subsistence, it is vulnerable to any reduction in the availability of its prey caused by pesticides. Most of the studies conducted on the impacts of pesticides in various environments report pesticide-induced changes in birdlife food resources (Avian Effects Dialogue Group 1994). Insectivorous birds are particularly vulnerable to agricultural and forest pesticides, which can significantly reduce insect prey populations. The reduced abundance of insects has been related to reduced survival, growth and reproduction among birds, as well as to changes in diet composition and quality (Avian Effects Dialogue Group 1994).
In Canada, the arrival of the West Nile Virus generated pesticide-based eradication programs in specific areas. In North America, the practice of spraying pesticides to control other insects in cities and towns is also becoming increasingly popular. It is known that pesticides can be transported over long distances in the air (Poissant 1999). A number of them are relatively volatile, so they evaporate quickly after being applied and disperse in the atmosphere (Poissant and Koprivnjak 1996). Chimney Swifts could be indirectly affected by these products, which cause an impoverishment of the aerial plankton by reducing populations of flying insects. Erskine (1992) expressed concern about aerial spraying against spruce budworm (Choristoneura fumiferana), which has taken place in New Brunswick from 1952 to 1993 and which could have reduced flying insect populations and affected Chimney Swift populations in the province.
In addition, insects that survive these pesticide applications become contaminated and are then eaten by swifts, which become contaminated in turn. There are some indications that Chimney Swifts, like birds of prey, can accumulate dangerous levels of pesticides. Chantler (1999) reports that high concentrations of DDE, a degradation product of DDT, have been found in the Guam Swiftlet (Aerodramus bartschi), another member of the swift family. Chantler (1999) also points out that, considering the position of swifts in the food chain and their extended longevity, it stands to reason that pesticides pose a risk to this family. Sick (1993) states that in Brazil various species of swifts are in decline, as are swallows and nightjars, all of them victims of the unrestricted use of pesticides. In Ontario, similar trends were observed in other aerial foragers such as swallows and nighthawks, which suggests that something may be happening to their food supply, namely insects (M. Cadman pers. comm.).
The recent fecundity and survival rates calculated for swifts in Quebec (Garneau and Gauthier, CWS-QC unpublished data) and Texas (Kyle and Kyle unpublished data) are comparable to those between 1930 and 1950, suggesting that DDT and its degradation products have not significantly reduced fecundity in Chimney Swifts as they did for other species.
There is intra- and interspecific competition among swift species, which can be significant in species nesting in cavities (Lack and Collins 1985). Disputes over the occupation of nesting sites have been reported between Common Swifts (Apus apus) and European Starlings (Sturnus vulgaris). In the case of the Chimney Swift, competition between adults could take on greater importance as suitable nesting sites become increasingly scarce. Intense competition could exclude birds from breeding due to a lack of available sites, but it could also affect the breeding success of the few lucky pairs, given the extra time required to defend and maintain their territory.
Since the Chimney Swift makes use of hollow trees in its South American winter range, the species is threatened by intensive logging and fires in the Amazon forest. The discovery of their wintering grounds in 1944 proves that the species uses hollow trees as roosting sites in South America (Brackbill 1950). In addition, after forests are cleared to create farmland, large quantities of pesticides are often used to control insects that are harmful to farm crops and humans. In some countries, very harmful pesticides that are banned in North America, such as DDT, are still being used. These products may be having a significant impact on the Chimney Swift, but no data are available.
Swifts roosting in a chimney do on occasion die from asphyxiation or are burned when the heat is turned on in cold weather (Deane 1908). This situation can cause the death of a large number of birds in a single roost. Musselman (1931) reported that 3000 to 5000 swifts died in October in a chimney in Illinois. At Lake Springfield, Illinois, Bohlen (1989) found 100 dead swifts killed by cars on a cold, rainy spring day when the birds were flying very low to catch insects.
Since Chimney Swifts spend most of their time in the air, coming down to nest and roost in fairly inaccessible locations such as chimneys or hollow trees, predation is likely a minimal threat to populations. However, Merlins have invaded Canadian urban centres in recent years and increasing attacks on swifts have been noted in Quebec. One Merlin preyed on swifts at a roost on the St-Jovite church in 1999 (M. Renaud, pers. comm.); during the period when the bird of prey was present, the swifts almost completely abandoned the roost.
According to Walker (1944), the Chimney Swift’s greatest single enemy is the weather. Cold can be very damaging for this insectivore. For instance, 109 birds were found dead on the hearth of the François Pilote Museum chimney in La Pocatière on May 23, 1990; apparently killed by low temperatures and snow (Aubry et al. 1990). Between 1999 and 2003, a video camera was used at an artificial nesting site in the Quebec urban area (Lévis). During many consecutive days of cold and rain, Garneau and Gauthier (CWS-QC unpublished data) observed that swifts never went out to forage. In addition, each time the temperature inside the chimney went below a threshold, swifts would leave the chimney, perhaps looking for a better site even though conditions remained unfavourable.
Precipitation can also kill birds indirectly. A steady drenching rain for two or three days may clear the air of insects and with the food supply gone, the birds are subject to starvation (Walker 1944). Cold, rainy weather in northern Europe is known to cause considerable mortality in swift and swallow populations (Elkins 1988). These unfavourable weather conditions, which reduce the number of airborne insects, indirectly cause the birds’ death. In addition to affecting the number of insects available, Chantler (1999) says that temperature and precipitation also have a major impact on swifts’ breeding success.
Heavy rain also detaches nests from chimney walls on occasion, which often destroys the eggs and nestlings (Dexter 1952b; 1960; 1981a). However, the young do sometimes survive and climb back up the wall, where the parents continue to feed them (Dexter 1952b; 1960; 1985).
Climate change will undoubtedly have consequences for birdlife. In a study on the evaluation of Quebec breeding birds’ vulnerability to climate change, Morneau et al. (1998) observed that of the 13 most climate-sensitive species, most of them were insectivorous neotropical migrants. The Chimney Swift was one of the 71 vulnerable species identified. Species that forage for insects on the wing are more sensitive to temperature variations as this variable directly affects insect abundance. Tropical insect species could expand their range northward. However, climate change could also be very damaging for some species by affecting the abundance and distribution of insects (Chantler 1999). Some data also suggest that climate warming will result in greater climate fluctuations. Temperature extremes, such as very cold springs or summers, could be catastrophic for aerial foragers such as the Chimney Swift. These extremes could also accelerate the degradation of the last remaining traditional chimneys. Warm weather and rain during the winter followed by extreme cold temperature can cause considerable damage to stone or brick structures. Water that infiltrates into the cement and bricks can cause erosion to the structure once it refreezes.
Climate change could also have an impact on the frequency, intensity and trajectories of hurricanes, which are particularly damaging to swift populations. They tend to occur during the fall migration period, and some, like Hurricane Wilma in October 2005, pushed more than 2000 Chimney Swifts north from staging areas further south. At least 700 Chimney Swifts were found dead in the Maritimes after Hurricane Wilma (D. Busby, unpub. data). According to the National Oceanic and Atmospheric Administration (NOAA 2005), the mean number of storms since 1995 is higher compared to the previous period (1970-1994).
The Chimney Swift appears on the list of birds which have been found dead and tested positive for the West Nile Virus in the United States (Center for Disease Control and Prevention, Division of Vector-Born Infectious Diseases 2003). There have been no cases of infection in Canada and the species is not on the list of birds retained for testing. The increase of insecticide spraying in response to this disease could also adversely affect insectivorous birds including the Chimney Swift (see Pesticides above).
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